1. Field of the Invention
This invention relates generally to digital-to-analog converters and, more particularly, to an improved bit switch for use in conjunction with the ladder network in a digital-to-analog converter (DAC).
2. Description of the Prior Art
Digital to analog converters which include a ladder network may employ a plurality of bit switches for switching a binary scaled bit current (I.sub.Bit) to an output bus or to ground depending upon the state of an input signal. That is, when the bit switch is turned on, it directs a pre-determined binary scaled current onto the output bus. Bit currents from the bit switches are then summed on the output bus to form an analog signal corresponding to the digital input signal.
One known type of bit switch includes a first differential transistor pair which receive an input signal and a reference voltage, and a second differential pair which switches the bit current between the output bus and ground. However, when using this arrangement, a rapid change in the input level is coupled through the base-collector capacitance of the input transistor of the first differential pair and affects the voltage at the base of one of the transistors in the second differential pair. Thus, the voltages at the bases of the second differential pair are not precisely differential voltages. This causes a glitch at the emitters of the second differential pair which, especially in the case of low bit currents, may take a considerable amount of time to settle out.